In optical transmission systems, Baud rates of 40 GBd and higher are used in order to increase the transmission capacity. When applying conventional coding and modulation methods like NRZ (non-return-to-zero) or RZ (return-to-zero), OOK (on-off-keying) high-speed transmitters and receivers revealing a respective high bandwidth are required. Further, in optical DWDM (dense wavelength division multiplex) systems revealing a channel spacing of 50 GHz, severe interference and distortion between channels occur if the Baud rate is increased to values greatly exceeding 40 GBd.
Optical duobinary (ODB) transmission technology, as e.g. described in US 2004/0086225 A1, is able to reduce the bandwidth requirements for optical transmitters and receivers. The optical duobinary signal that is transmitted over the optical transmission link reveals a reduced bandwidth as compared to a standard NRZ OOK optical signal. Thus, channel interference and distortion is avoided and the costs for lower bandwidth transmitters and receivers are reduced. Further, optical duobinary transmission is less sensitive to chromatic dispersion as neighboring “1” bits or symbols separated by a single “0” bit or symbol, only, are transmitted at a 180 degree phase shift with respect to the electric field vector, so that interfering portions of corresponding neighboring bits or symbols at least partially cancel each other.
At this point, it shall be noted that the symbol rate of an ODB signal is equal to the bit rate of the binary digital transmit signal that has been pre-coded and duobinary encoded for creating the ODB signal.
However, at the receiver side, the bandwidth requirement in case of ODB transmission is still in the region of the bit rate or symbol rate. Lower bandwidth and therefore cheaper components can only be used together with electronic post-processing, e.g. maximum likelihood sequence estimation (MLSE). However, this increases the complexity and thus the costs of the electronic equipment.